System and method for communicating data using constant envelope orthogonal Walsh modulation with channelization

ABSTRACT

A radio device includes a transmitter having a modulator for generating M-PAM communications symbols containing communications data. A Fast Walsh Transform circuit orthogonally encodes and band-spreads a communications symbol using the Fast Walsh Transform. A frequency modulation circuit frequency modulates the communications symbols wherein a constant envelope orthogonal Walsh modulated communications signal is generated having a plurality of orthogonal waveforms each forming a separate Walsh communications channel.

FIELD OF THE INVENTION

The present invention relates to communications systems and, moreparticularly, the present invention relates to communications systemsthat use constant envelope (CE) waveforms and orthogonal modulation.

BACKGROUND OF THE INVENTION

Some multi-band or other tactical radios operate in the high frequency(HP), very high frequency (VHF) (for satellite communications), andultra high frequency (UHF) bands. The frequency range of thesemulti-band tactical radios can operate over about 2 through about 512MHz. Next generation radios will probably cover about 2.0 to about 2,000MHz (or higher) to accommodate high data rate, higher bandwidthwaveforms and less crowded frequency bands. In the HF frequency band thetransmit mode is governed by standards such as MIL-STD-188-141B, whiledata modulation/demodulation is governed by standards such asMIL-STD-188-110B, the disclosures which are incorporated by reference intheir entirety.

UHF standards, on the other hand, provide different challenges over the225 to about 512 MHz frequency range, including short-haul line-of-sight(LOS) communication and satellite communications (SATCOM) and cable.This type of propagation can be obtained through different weatherconditions, foliage and other obstacles making UHF SATCOM anindispensable communications medium for many agencies. Differentdirectional antennas can be used to improve antenna gain and improvedata rates on the transmit and receive links. This type of communicationis typically governed in one example by MIL-STD-188-181B, the disclosurewhich is incorporated by reference in its entirety. This standardprovides a family of constant and non-constant amplitude waveforms foruse over satellite links.

The joint tactical radio system (JTRS) is one example of a system thatimplements some of these standards and has different designs that useoscillators, mixers, switchers, splitters, combiners and power amplifierdevices to cover different frequency ranges. The modulation schemes usedfor these types of systems can occupy a fixed bandwidth channel at afixed frequency spectrum. These systems usually utilize a memorylessmodulation, such as phase shift keying (PSK), amplitude shift keying(ASK), frequency shift keying (FSK), quadrature amplitude modulation(QAM), or modulations with memory such as continuous phase modulation(CPM) and may sometimes combine them with a convolutional or other typeof forward error correction (FEC) code. Minimum shift keying (MSK) andGaussian minimum shift keying (GSMK) (together referred to as MSK orGMSK) are a form of continuous phase modulation used in the GlobalSystem for Mobile communications (GSM) and can be used with suchsystems. The circuits used for implementing the MSK waveform couldinclude a continuous phase frequency shift keying (FSK) modulator.

Briefly, an MSK modulated signal can be considered as two combinedorthogonal signals or channels that are 90 degrees out of phase witheach other. Typically, each phase reversal is keyed to representalternate bits of a binary signal that is to be transmitted. Each keyedpulse period could have a duration of a two bit period that is staggeredby a one bit period, and when binary data is used to modulate eachchannel, the channels can be amplitude modulated with a positive ornegative one-half wave sinusoid and combined by addition. Because thesine shaped envelopes of the two channels are 90 degrees out of phasewith each other, the sum of the two channels results in a signal with aconstant envelope, which could be amplified by non-linear class-Camplifiers and transmitted. A Gaussian filter having a Gaussian impulseresponse can be used for prefiltering symbols prior to any continuousphase modulation, thus forming a Gaussian minimum shift keying.

Spread Spectrum (SS) modulation spreads a waveform in frequency andtypically provides robust data performance. SS modulation can useunderlying orthogonal spreading sequences (i.e., Walsh Hadamardsequences) or pseudo-orthogonal spreading sequences (i.e., sequencesobtained from maximum length shift-register sequences, shortened Walshsymbols, overloaded Walsh symbols, gold codes, or others).

Typically, two signals x (t) and y (t) are orthogonal when the averageof their product x (t) y (t) equals zero. x (t) and y (t) can be randomor Pseudo-random Noise (PN) signals and can be near-orthogonal (i.e.,pseudo-orthogonal) with their products being zero in the mean, butsometimes not identically zero for all signal pairs. Any signalstransmitted in these spread spectrum system are typically received anddecoded and correlated in matched filters and/or signal processors thatcorrelate a correlation function between a reference signal s(t) and thereceived signal r(t).

One common example of SS orthogonal data modulation is M-ary Walshmodulation. For example, IS-95 uses a 64-ary orthogonal Walsh modulationcombined with PSK to send 6 bits of information. By definition, Walshsymbols are a group of M vectors that contain M binary elements in whichevery Walsh symbol of a given length is orthogonal to all other Walshsymbols of that length and all inverses of the other Walsh symbols ofthat length. For example, some systems use Walsh symbols having 64 chipsto identify the logic channels. On both the forward and reverse channelsthe Walsh symbols have orthogonality. Walsh symbols can be producedusing a simple iterative technique utilizing a base Walsh matrix.

In some current high performance radio network systems, it has beenfound that better performance can be achieved when robust, burstwaveforms are used for the control, status and lower data rate datamessages within the communications network. Many radio network systems,for example, such as manufactured by the assignee of the presentinvention, Harris Corporation of Melbourne, Fla., have used orthogonalWalsh modulation schemes to achieve the necessary level of robustness.Most radio frequency (RF) power amplifiers are peak power limited,however. For example, average power transmitted for a filtered SS phaseshift keying (PSK) waveform can be several decibels (dB) less than thepeak power capability of an RF amplifier because of the back-offrequired to accommodate the waveform's peak-to-average ratio. A constantamplitude waveform advantageously addresses this issue, but it is alsodesirable to maintain robustness and provide an adequate level ofcapacity (bps/Hz).

As known to many skilled in the art, constant amplitude or envelopewaveforms are typically required for class C or class E amplifiers usedin small handheld or personal radios. These types of power amplifiersare more efficient than the linear class A and class B amplifiers usedin other radios. Class C and Class E power amplifiers are generally moreefficient than linear RF power amplifiers (i.e., class A, AB, or B). Thedesign challenges when using these types of waveforms is to maintain theconstant envelope (CE) while also providing an adequate level ofcapacity (bps/Hz).

Constant amplitude (or envelope) waveforms are becoming increasinglyimportant for handheld communications devices or radio devices. GaussianMinimum Shift Keying (GMSK) and similar waveform variations used bythose skilled in the art often are the basis for many radio waveformsused in such devices, but their communications are limited typically toone (1) bps/Hz. Performance issues, however, often cancel any gainsresulting from the use of constant envelope waveforms, especially insmaller battery operated radios often used in a military or somecommercial environments.

SUMMARY OF THE INVENTION

A radio device includes a transmitter having a modulator for generatingM-PAM communications symbols containing communications data. A FastWalsh Transform circuit orthogonally encodes and band-spreads acommunications symbol using the Fast Walsh Transform. A frequencymodulation circuit frequency modulates the communications symbolswherein a constant envelope orthogonal Walsh modulated communicationssignal is generated having a plurality of orthogonal waveforms eachforming a separate Walsh communications channel.

At least one of the separate Walsh communications channels comprises abroadcast channel for transmitting a communications signal containingcommunications data to a plurality of receivers. At least one of theseparate Walsh communications channels can be formed of at least one ofa voice and data channel or a control channel. A substantial portion ofthe transmit power from the transmitter unit can be allocated to atleast one Walsh communications channel.

In another aspect a square root raised cosine filter is operative beforethe frequency modulation circuit and band-limits a signal containing thecommunications symbols. A clipping device can be used to reduce the peakto average power ratio of the signal input to frequency modulationcircuit. The modulator could be formed for generating binary ormulti-level M-PAM communication signals. An up-sampling circuit canreceive the signal from the Fast Walsh Transform circuit for increasingthe sampling rate on the signal containing the communications symbols.

In another aspect, at least one receiver is located remote from thetransmitter and receives the constant envelope Walsh modulatedcommunications signal and includes one of at least a phase demodulatorcircuit for phase demodulating the received signal and a phase unwrapcircuit for phase unwrapping the received signal. A FWT circuit performsa Fast Walsh Transform as a demodulator matched filter to obtain anycommunications data that had been transmitted from the transmitter andcontained within the at least one received communications channel. Thereceiver can include a multi-user detection circuit for applyingstandard or iterative multi-user detection algorithms to the receivedsignal. The receiver can also be formed as a feedback loop from themulti-user detection circuit for iterative processing.

A method aspect is also set forth.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome apparent from the detailed description of the invention whichfollows, when considered in light of the accompanying drawings in which:

FIG. 1 is a block diagram showing basic processing components used in atransmitter for the constant radius orthogonal Walsh modulation inaccordance with a non-limiting example of the present invention.

FIG. 2 is a block diagram showing basic processing components used in areceiver for the constant radius orthogonal Walsh modulation inaccordance with a non-limiting example of the present invention.

FIG. 3 is a graph showing a spectrum comparison and results fordifferent signal examples including the constant radius orthogonal Walshmodulation in accordance with a non-limiting example of the presentinvention.

FIG. 4 is a block diagram of an example of a communications system thatcan be used and modified to work with constant radius orthogonal Walshmodulation in accordance with a non-limiting example of the presentinvention.

FIG. 5 is a high-level block diagram showing basic components that canbe used and modified to work with the constant radius orthogonal Walshmodulation in accordance with a non-limiting example of the presentinvention.

FIG. 6 is a perspective view of a portable wireless communicationsdevice as a handheld radio that could incorporate the communicationssystem and radio as modified to work with the constant radius orthogonalWalsh modulation in accordance with a non-limiting example of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Different embodiments will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsare shown. Many different forms can be set forth and describedembodiments should not be construed as limited to the embodiments setforth herein. Rather, these embodiments are provided so that thisdisclosure will be thorough and complete, and will fully convey thescope to those skilled in the art. Like numbers refer to like elementsthroughout.

It should be appreciated by one skilled in the art that the approach tobe described is not limited for use with any particular communicationstandard (wireless or otherwise) and can be adapted for use withnumerous wireless (or wired) communications standards such as EnhancedData rates for GSM Evolution (EDGE), General Packet Radio Service (GPRS)or Enhanced GPRS (ZGPRS), extended data rate Bluetooth, Wideband CodeDivision Multiple Access (WCDMA), Wireless LAN (WLAN), Ultra Wideband(UWB), coaxial cable, radar, optical, etc. Further, the invention is notlimited for use with a specific physical layer (PHY) device or radiotype but is applicable to other compatible technologies as well.

Throughout this description, the term communications device is definedas any apparatus or mechanism adapted to transmit, receive or transmitand receive data through a medium. The communications device may beadapted to communicate over any suitable medium such as RF, wireless,infrared, optical, wired, microwave, etc. In the case of wirelesscommunications, the communications device may comprise an RFtransmitter, RF receiver, RF transceiver or any combination thereof.Wireless communication involves: radio frequency communication;microwave communication, for example long-range line-of-sight via highlydirectional antennas, or short-range communication; and/or infrared (IR)short-range communication. Applications may involve point-to-pointcommunication, point-to-multipoint communication, broadcasting, cellularnetworks and other wireless networks.

As will be appreciated by those skilled in the art, a method, dataprocessing system, or computer program product can embody differentexamples in accordance with a non-limiting example of the presentinvention. Accordingly, these portions may take the form of an entirelyhardware embodiment, an entirely software embodiment, or an embodimentcombining software and hardware aspects. Furthermore, portions may be acomputer program product on a computer-usable storage medium havingcomputer readable program code on the medium. Any suitable computerreadable medium may be utilized including, but not limited to, staticand dynamic storage devices, hard disks, optical storage devices, andmagnetic storage devices.

The description as presented below can apply with reference to flowchartillustrations of methods, systems, and computer program productsaccording to an embodiment of the invention. It will be understood thatblocks of the illustrations, and combinations of blocks in theillustrations, can be implemented by computer program instructions.These computer program instructions may be provided to a processor of ageneral purpose computer, special purpose computer, or otherprogrammable data processing apparatus to produce a machine, such thatthe instructions, which execute via the processor of the computer orother programmable data processing apparatus, implement the functionsspecified in the block or blocks.

These computer program instructions may also be stored in acomputer-readable memory that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablememory result in an article of manufacture including instructions whichimplement the function specified in the flowchart block or blocks. Thecomputer program instructions may also be loaded onto a computer orother programmable data processing apparatus to cause a series ofoperational steps to be performed on the computer or other programmableapparatus to produce a computer implemented process such that theinstructions which execute on the computer or other programmableapparatus provide steps for implementing the functions specified in theflowchart block or blocks.

FIG. 1 is a block diagram showing basic processing modules of a radiodevice shown by the solid outline and the transmitter unit andillustrated at 10 and used for the constant radius orthogonal Walshmodulation in accordance with a non-limiting example. The term constantenvelope can also be applied since they convey the same idea (i.e. aconstant envelope waveform is displayed as a circle with constant radiuswhen the amplitude of waveform is plotted as the real part of waveformversus the imaginary part of waveform). The radio device typicallyincludes a receiver unit such as shown in FIG. 2. The transmitter unit10 as shown in FIG. 1 in one non-limiting example is based on ConstantEnvelope-Orthogonal Frequency Division Multiplexing (CE-OFDM) withoutthe OFDM portion. By using a Fast Walsh Transform, the data is spreadmore evenly over all frequencies instead of having discrete tones spreadby the frequency modulator as in a CE-OFDM system. The transmitter unit10 preferably uses M-PAM (Pulse Amplitude Modulation) as a type of M-aryPAM in which a M-PAM (or other real modulation scheme) module modulatesusing M-ary Pulse Amplitude Modulation in which an output is a basebandrepresentation of the modulated signal. As is known to those skilled inthe art, the M-ary number parameter, M, is the number of points in asignal constellation and is typically an even integer and a power of 2.Baseband M-ary pulse amplitude modulation (PAM) typically uses a defaultsignal constellation block and maps an integer m between 0 and M−1 to avalue 2m−1−M.

The modulator selects and transmits one of M waveforms in response tolog₂M bits. The waveforms typically differ from each other only inamplitude. With M-PAM modulation, some skilled in the art refer to thedifferent waveform amplitudes using the set of integers {A_(m)} with{A_(m)}=2m−1−M, m=1, 2, . . . , M. {A_(m)}, which can be referred to asthe symbol alphabet. A simple case occurs for M=2, e.g., where thesymbol alphabet is {−1, +1}. For M=4 and the corresponding symbolalphabet is {−3, −1, +1, +3}.

The transmitter unit 10 includes basic transmitter circuits and uses aFast Walsh Transform (FWT) to encode orthogonally and frequency spreadtransmit symbols. This provides a higher capacity than standard GMSKtechniques. Typical approaches for CE-OFDM are tone based, but inaccordance with a non-limiting example, an FM modulator is used tospread tones when transmitted. Of course, any tone based approach maynot be as advantageous as desired, but in some instances, spreading oftones with FM modulation is adequate. Walsh functions, however, offeranother level of spreading using Walsh algorithms for processing toincrease robustness and to simplify the computational processingrequired. Thus, the transmit system spreads over more frequency and ismore efficient to implement with the Fast Walsh Transform.

The transmitter unit 10 as shown in FIG. 1 includes various functionalmodules forming a part of the entire transmitter circuit. The unit 14converts the data bits to one of the M M-PAM signals and can be binaryor multi-level. The Fast Walsh Transform (FWT) generates the waveformand is computationally efficient such that only +/− factors are requiredin a circuit that could be implemented using a Field Programmable GateArray (FPGA). The data bits can be spread over the frequency range. Thisis advantageous compared to modulating a single tone as is typical withmany current CE-OFDM systems. A square root raised cosine filter can beused to limit the bandwidth of the waveform at the input to the FMmodulator. A clipping circuit can be used to clip the waveform andreduce the peak to average power ratio of waveform prior to the phasemodulator.

As illustrated by the various modules shown with the transmitter unit10, a signal is input having its communication data into aserial-to-parallel conversion module (s/p) 12 and passed as paralleldata into an M-PAM modulator 14 that generates the M-PAM symbols. Thissignal is then processed by a Fast Walsh Transform module 16 andup-sampled 4 (four) to about 8 (eight) times in one non-limiting exampleof the up-sampling circuit 18. The guard time, also called guardinterval, is added by appropriate guard circuit 19, which could bedifferent areas of the circuit. This circuit cyclically extends theoutput of the Fast Walsh Transform by a guard time. The signal is shapedwith a Square Root Raised Cosine Filter 20 and clipped within a clippercircuit 22 and passed into the FM modulator 24. The clipper circuit 22is an alternative or optional circuit. Once the signal is FM modulated,it is transmitted as indicated by an antenna at 26. It should beunderstood that in comparison to a more conventional system thatprocesses communications data into a conventional CE-OFDM signal, on theother hand, these conventional systems for producing such CE-OFDMsignals typically do not include up-sampling and do not use a squareroot raised cosine filter. These conventional systems also do nottypically incorporate a Fast Walsh Transform (FWT) with some FMmodulation.

FIG. 2 shows a receiver unit 40 and its basic processing components usedfor basic components of a receiver and receiving a signal transmitted bythe transmitter unit 10 shown in FIG. 1 and processing such signal toobtain the transmitted communications data. As illustrated and explainedin greater detail below, a demodulator circuit is operative also as ademodulator matched filter and Fast Walsh Transform circuit. Demodulatorperformance may benefit from standard or iterative multi-user detection(MUD) algorithms.

As illustrated, the communications signal is received through an antenna42 as part of a receiver for a circuit forming the receiver unit 40 anddown converted with an appropriate down-convert circuit 44. It isoversampled 4 to about 8 times with oversample circuit 44. Guard timeremoval can occur at guard circuit 43. A low pass filter 46 is operativeas a band limit filter and the Frequency Domain Equalizer (FD EQU) 44incorporates both a Fast Fourier Transform (FFT) and an Inverse FastFourier Transform (IFFT). Thus, in the receiver unit 40 as illustratedand explained, there is processing to the frequency domain and then backto the time domain.

The Arc-Tan circuit 50 phase demodulates the received signal and a phaseunwrap circuit unwraps the output of phase demodulator and removes phasediscontinuities from the signal, followed by processing within a SquareRoot RC filter 52 with decimation of about 2×−1×. The Fast WalshTransform circuit 54 undoes what had been accomplished at the transmitsystem side concerning the Walsh modulation and is typically an inversetransform (note that an inverse Walsh transform is a fast Walshtransform). The MUD-IMUD circuit 56 operates on the received andprocessed signal for the multi-user detection (MUD) and iterativemulti-user detection (IMUD) and is followed by parallel-to-serialprocessing within parallel/serial converter circuit 58.

The Fast Walsh Transform 54 is operative using adds and subtracts andthere are no multiples, and thus, this circuit function can be readilyimplemented within a Field Programmable Gate Array (FPGA). Because thereis an FFT and IFFT, the system benefits from the Fast Walsh Transform.The Frequency Domain Equalizer 48 and Multi-User Detection circuit 56can be optional.

If Forward Error Correction (FEC) is used, there can be an iterativeloop from the MUD-IMUD circuit 56 back to the FWT circuit 54 or from theMUD-IMUD circuit back to the Frequency Domain Equalizer 48 as shown bythe dashed lines, indicating this is option depending on whether sometype of Forward Error Correction (FEC) is used.

The transmitter unit 10 and receiver unit 40 as described have variousadvantages. It is possible to implement either system in FieldProgrammable Gate Array (FPGA) circuits. The systems build on orthogonalmodulation and associated IP research, and provide a higher capacitywhile maintaining a constant ratio frequency (RF) envelope. The FastWalsh Transform at the receiver unit 40 compensates for non-lineareffects in any phase unwrapped circuit. The system can generate a familyof waveforms providing different levels of robustness or covertness.

It should be understood that a Walsh function can form an orthogonalbasis of square-integrable functions on a unit interval and can take thevalues −1 and 1 only, such as on sub-intervals. These Walsh functionscan be used to perform a Hadamard transform, similar to the manner anyorthogonal sinusoids are used to perform a Fourier transform. Typically,the order of this function is 2^(s), where ^(s) is an integer meaningthere are 2^(s) time intervals in which a value can be −1 or 1. Thus, alist of 2^(s) Walsh functions make the Hadamard matrix.

It should be understood that the transmitter and receiver can use theconstant envelope orthogonal Walsh modulation using a Fast WalshTransform (FWT) as described to encode orthogonally and band spreadtransmit symbols and provide higher capacity than standard GMSK.

It is also possible to transmit data using each orthogonal waveform as aseparate channel. For example, one channel (i.e. one Walsh symbol orbin) could be used to transmit to numerous different receivers such as abroadcast channel. Another channel could be a voice channel and thecomposite signal of all the channels would still be constant envelope(CE). Thus, sending information on different channels for differentusers would still provide the desirable property of constant envelopeand obtain power efficiency. Thus, Walsh bins can be used as different“channels” for control, voice, data and broadcast. Multiple channelscould be used for one service (i.e. control channel) to increaserobustness of channel. Even when sending different data to many users,the waveform is a constant envelope. Any received symbols in the systemare processed for multiple orthogonal channels, e.g., Walsh bins. Thesystem includes a constant radius (i.e., envelope) orthogonal Walsh(CROW) modulation and takes advantage of the channelized properties ofCROW and CE-OFDM to create multiple streams of data such as thebroadcast channel, digital voice channel, control channel, diversitychannels, and similar channels. If there is only one channel,substantially all power or a substantial portion of the power from thetransmitter could go to that user, and the waveform is still constantenvelope (CE). If there are multiple channels, power is spread among allchannels and users, but the waveform is still constant envelope, e.g.,amplitude. This is an advantageous design.

FIG. 3 illustrates a graph showing different spectrum comparison resultsand showing in the outer line at 70 the MASK as the limit for transmitpower in one non-limiting aspect. As shown on the graph, some of theresulting graph lines are spread and more flat, while others are morerounded and drop off, thus indicating and showing the greater energyinside the band. It should be understood that the system provides acombination of real orthogonal modulation with FM modulation andopportunely uses the Fast Walsh Transform (FWT) modulator and a familyof constant envelope (CE) waveforms in an advantageous manner.

The transmit and receive systems are applicable to Secure Personal Radio(SPR) radios such as the SPR RF-7800S radio manufactured and sold byHarris Corporation. The systems are also applicable to an OperationalTests-Tactical Engagement System (OT-TES) customers and applicable toMobile User Objective System (MUOS) systems that typically include anarray of satellites that use the Ultra High Frequency (UHF) (300 MHz to3 GHz frequency range) satellite communications (SATCOM) systems, forexample, used by the Department of Defense. Such systems can be builtupon the commercial third generation (3G) Wideband Code DivisionMultiple Access (WCDMA) cellular phones as used with differentcommunications systems, including military systems with UHF SATCOM radiosystems and geosynchronous satellites instead of cell towers. It is alsoapplicable to wideband networking systems such as the WNW, SRW and ANW2such as used by Harris Corporation. As noted before, Walsh bins can beused as different “channels” for control, voice, data and broadcast.

For purposes of description, some further information on coding,interleaving, and an exemplary wireless, mobile radio communicationssystem that includes ad-hoc capability and can be modified for use isset forth. This example of a communications system that can be used andmodified for use with the present invention is now set forth with regardto FIGS. 4 and 5. FIG. 4 shows a number of radio devices that could betransmitters and receivers.

An example of a radio that could be used with such system and method isa Falcon™ III radio manufactured and sold by Harris Corporation ofMelbourne, Fla. This type of radio can support multiple wavebands form30 MHz up to 2 GHz, including L-band SATCOM and MANET. The waveforms canprovide secure IP data networking. It should be understood thatdifferent radios can be used, including software defined radios that canbe typically implemented with relatively standard processor and hardwarecomponents. One particular class of software radio is the Joint TacticalRadio (JTR), which includes relatively standard radio and processinghardware along with any appropriate waveform software modules toimplement the communication waveforms a radio will use. JTR radios alsouse operating system software that conforms with the softwarecommunications architecture (SCA) specification (seewww.jtrs.saalt.mil), which is hereby incorporated by reference in itsentirety. The SCA is an open architecture framework that specifies howhardware and software components are to interoperate so that differentmanufacturers and developers can readily integrate the respectivecomponents into a single device.

The Joint Tactical Radio System (JTRS) Software Component Architecture(SCA) defines a set of interfaces and protocols, often based on theCommon Object Request Broker Architecture (CORBA), for implementing aSoftware Defined Radio (SDR). In part, JTRS and its SCA are used with afamily of software re-programmable radios. As such, the SCA is aspecific set of rules, methods, and design criteria for implementingsoftware re-programmable digital radios.

The JTRS SCA specification is published by the JTRS Joint Program Office(JPO). The JTRS SCA has been structured to provide for portability ofapplications software between different JTRS SCA implementations,leverage commercial standards to reduce development cost, reducedevelopment time of new waveforms through the ability to reuse designmodules, and build on evolving commercial frameworks and architectures.

The JTRS SCA is not a system specification, as it is intended to beimplementation independent, but a set of rules that constrain the designof systems to achieve desired JTRS objectives. The software framework ofthe JTRS SCA defines the Operating Environment (OE) and specifies theservices and interfaces that applications use from that environment. TheSCA OE comprises a Core Framework (CF), a CORBA middleware, and anOperating System (OS) based on the Portable Operating System Interface(POSIX) with associated board support packages. The JTRS SCA alsoprovides a building block structure (defined in the API Supplement) fordefining application programming interfaces (APIs) between applicationsoftware components.

The JTRS SCA Core Framework (CF) is an architectural concept definingthe essential, “core” set of open software Interfaces and Profiles thatprovide for the deployment, management, interconnection, andintercommunication of software application components in embedded,distributed-computing communication systems. Interfaces may be definedin the JTRS SCA Specification. However, developers may implement some ofthem, some may be implemented by non-care applications (i.e., waveforms,etc.), and some may be implemented by hardware device providers.

For purposes of description only, a brief description of an example of acommunications system that includes communications devices thatincorporate the simultaneous wideband and narrowband communications inaccordance with a non-limiting example, is described relative to anon-limiting example shown in FIG. 7. This high-level block diagram of acommunications system includes a base station segment and wirelessmessage terminals that could be modified for use with the presentinvention. The base station segment includes a VHF radio 360 and HFradio 362 that communicate and transmit voice or data over a wirelesslink to a VHF net 364 or HF net 366, each which include a number ofrespective VHF radios 368 and HF radios 370, and personal computerworkstations 372 connected to the radios 368,370. Ad-hoc communicationnetworks 373 are interoperative with the various components asillustrated. The entire network can be ad-hoc and include source,destination and neighboring mobile nodes. Thus, it should be understoodthat the HF or VHF networks include HF and VHF net segments that areinfrastructure-less and operative as the ad-hoc communications network.Although UHF and higher frequency radios and net segments are notillustrated, these could be included.

The radio can include a demodulator circuit 362 a and appropriateconvolutional encoder circuit 362 b, block interleaver 362 c, datarandomizer circuit 362 d, data and framing circuit 362 e, modulationcircuit 362 f, matched filter circuit 362 g, block or symbol equalizercircuit 362 h with an appropriate clamping device, deinterleaver anddecoder circuit 362 i modem 362 j, and power adaptation circuit 362 k asnon-limiting examples. A vocoder circuit 362 l can incorporate thedecode and encode functions and a conversion unit could be a combinationof the various circuits as described or a separate circuit. A clockcircuit 362 m can establish the physical clock time and through secondorder calculations as described below, a virtual clock time. The networkcan have an overall network clock time. These and other circuits operateto perform any functions necessary for the present invention, as well asother functions suggested by those skilled in the art. Other illustratedradios, including all VHF (or UHF) and higher frequency mobile radiosand transmitting and receiving stations can have similar functionalcircuits. Radios could range from 30 MHz to about 2 GHz as non-limitingexamples.

The base station segment includes a landline connection to a publicswitched telephone network (PSTN) 380, which connects to a PABX 382. Asatellite interface 384, such as a satellite ground station, connects tothe PABX 382, which connects to processors forming wireless gateways 386a, 386 b. These interconnect to the VHF radio 360 or HF radio 362,respectively. The processors are connected through a local area networkto the PABX 382 and e-mail clients 390. The radios include appropriatesignal generators and modulators.

An Ethernet/TCP-IP local area network could operate as a “radio” mailserver. E-mail messages could be sent over radio links and local airnetworks using STANAG-5066 as second-generation protocols/waveforms, thedisclosure which is hereby incorporated by reference in its entiretyand, of course, preferably with the third-generation interoperabilitystandard: STANAG-4538, the disclosure which is hereby incorporated byreference in its entirety. An interoperability standard FED-STD-1052,the disclosure which is hereby incorporated by reference in itsentirety, could be used with legacy wireless devices. Examples ofequipment that can be used in the present invention include differentwireless gateway and radios manufactured by Harris Corporation ofMelbourne, Fla. This equipment could include RF5800, 5022, 7210, 5710,5285 and PRC 117 and 138 series equipment and devices as non-limitingexamples.

These systems can be operable with RF-5710A high-frequency (HF) modemsand with the NATO standard known as STANAG 4539, the disclosure which ishereby incorporated by reference in its entirety, which provides fortransmission of long distance radio at rates up to 9,600 bps. Inaddition to modem technology, those systems can use wireless emailproducts that use a suite of data-link protocols designed and perfectedfor stressed tactical channels, such as the STANAG 4538 or STANAG 5066,the disclosures which are hereby incorporated by reference in theirentirety. It is also possible to use a fixed, non-adaptive data rate ashigh as 19,200 bps with a radio set to ISB mode and an HF modem set to afixed data rate. It is possible to use code combining techniques andARQ.

A communications system that incorporates communications devices can beused in accordance with non-limiting examples of the present inventionand is shown in FIG. 5. A transmitter is shown at 391 and includes basicfunctional circuit components or modules, including a forward errorcorrection encoder 392 a that includes a puncturing module, which couldbe integral to the encoder or a separate module. The decoder 392 a andits puncturing module includes a function for repeating as will beexplained below. Encoded data is interleaved at an interleaver 392 b,for example, a block interleaver, and in many cases modulated atmodulator 392 c. This modulator can map the communications data intodifferent symbols based on a specific mapping algorithm to form acommunications signal. For example, it could form Minimum Shift Keyingor Gaussian Minimum Shift Keying (MSK or GMSK) symbols. Other types ofmodulation could be used in accordance with non-limiting examples of thepresent invention. Up-conversion and filtering occurs at an up-converterand filter 392 d, which could be formed as an integrated module orseparate modules. Communications signals are transmitted, for example,wirelessly to receiver 393.

At the receiver 393, down conversion and filtering occurs at a downconverter and filter 394 a, which could be integrated or separatemodules. The signal is demodulated at demodulator 394 b anddeinterleaved at deinterleaver 394 c. The deinterleaved data (i.e., bitsoft decisions) is decoded and depunctured (for punctured codes),combined (for repeated codes) and passed through (for standard codes) atdecoder 394 d, which could include a separate or integrated depuncturingmodule. The system, apparatus and method can use different modules anddifferent functions. These components as described could typically becontained within one transceiver.

It should be understood, in one non-limiting aspect of the presentinvention, a rate 1/2, K=7 convolutional code can be used as an industrystandard code for forward error correction (FEC) during encoding. Forpurposes of understanding, a more detailed description of basiccomponents now follows. A convolutional code is an error-correctingcode, and usually has three parameters (n, k, m) with n equal to thenumber of output bits, k equal to the number of input bits, and m equalto the number of memory registers, in one non-limiting example. Thequantity k/n could be called the code rate with this definition and is ameasure of the efficiency of the code, K and n parameters can range from1 to 8, m can range from 2 to 10, and the code rate can range from 1/8to 7/8 in non-limiting examples. Sometimes convolutional code chips arespecified by parameters (n, k, L) with L equal to the constraint lengthof the code as L=k (m−1). Thus, the constraint length can represent thenumber of bits in an encoder memory that would affect the generation ofn output bits. Sometimes the letters may be switched depending on thedefinitions used.

The transformation of the encoded data is a function of the informationsymbols and the constraint length of the code. Single bit input codescan produce punctured codes that give different code rates. For example,when a rate 1/2 code is used, the transmission of a subset of the outputbits of the encoder can convert the rate 1/2 code into a rate 2/3 code.Thus, one hardware circuit or module can produce codes of differentrates. Punctured codes allow rates to be changed dynamically throughsoftware or hardware depending on channel conditions, such as rain orother channel impairing conditions.

An encoder for a convolutional code typically uses a look-up table forencoding, which usually includes an input bit as well as a number ofprevious input bits (known as the state of the encoder), the table valuebeing the output bit or bits of the encoder. It is possible to view theencoder function as a state diagram, a tree diagram or a trellisdiagram.

Decoding systems for convolutional codes can use 1) sequential decoding,or 2) maximum likelihood decoding, also referred to as Viterbi decoding,which typically is more desirable. Sequential decoding allows bothforward and backward movement through the trellis. Viterbi decoding asmaximum likelihood decoding examines a receive sequence of given length,computes a metric for each path, and makes a decision based on themetric.

Puncturing convolutional codes is a common practice in some systems andis used in accordance with non-limiting examples of the presentinvention. It should be understood that in some examples a puncturedconvolutional code is a higher rate code obtained by the periodicelimination of specific code bits from the output of a low rate encoder.Punctured convolutional code performance can be degraded compared withoriginal codes, but typically the coding rate increases.

Some of the basic components that could be used as non-limiting examplesof the present invention include a transmitter that incorporates aconvolutional encoder, which encodes a sequence of binary input vectorsto produce the sequence of binary output vectors and can be definedusing a trellis structure. An interleaver, for example, a blockinterleaver, can permute the bits of the output vectors. The interleaveddata would also be modulated at the transmitter (by mapping to transmitsymbols) and transmitted. At a receiver, a demodulator demodulates thesignal.

A block deinterleaver recovers the bits that were interleaved. A Viterbidecoder could decode the deinterleaved bit soft decisions to producebinary output data.

Often a Viterbi forward error correction module or core is used thatwould include a convolutional encoder and Viterbi decoder as part of aradio transceiver as described above. For example if the constraintlength of the convolutional code is 7, the encoder and Viterbi decodercould support selectable code rates of 1/2, 2/3, 3/4, 4/5, 5/6, 6/7, 7/8using industry standard puncturing algorithms.

Different design and block systems parameters could include theconstraint length as a number of input bits over which the convolutionalcode is computed, and a convolutional code rate as the ratio of theinput to output bits for the convolutional encoder. The puncturing ratecould include a ratio of input to output bits for the convolutionalencoder using the puncturing process, for example, derived from a rate1/2 code.

The Viterbi decoder parameters could include the convolutional code rateas a ratio of input to output bits for the convolutional encoder. Thepuncture rate could be the ratio of input to output bits for theconvolutional encoder using a puncturing process and can be derived froma rate 1/2 mother code. The input bits could be the number of processingbits for the decoder. The Viterbi input width could be the width ofinput data (i.e. soft decisions) to the Viterbi decoder. A metricregister length could be the width of registers storing the metrics. Atrace back depth could be the length of path required by the Viterbidecoder to compute the most likely decoded bit value. The size of thememory storing the path metrics information for the decoding processcould be the memory size. In some instances, a Viterbi decoder couldinclude a First-In/First-Out (FIFO) buffer between depuncture andViterbi function blocks or modules. The Viterbi output width could bethe width of input data to the Viterbi decoder.

The encoder could include a puncturing block circuit or module as notedabove. Usually a convolutional encoder may have a constraint length of 7and take the form of a shift register with a number of elements, forexample, 6. One bit can be input for each clock cycle. Thus, the outputbits could be defined by a combination of shift register elements usinga standard generator code and be concatenated to form an encoded outputsequence. There could be a serial or parallel byte data interface at theinput. The output width could be programmable depending on the puncturedcode rate of the application.

A Viterbi decoder in non-limiting examples could divide the input datastream into blocks, and estimate the most likely data sequence. Eachdecoded data sequence could be output in a burst. The input andcalculations can be continuous and require four clock cycles for everytwo bits of data in one non-limiting example. An input FIFO can bedependent on a depuncture input data rate.

It should also be understood that the present invention is not limitedto convolutional codes and similar FEC, but also turbo codes could beused as high-performance error correction codes or low-densityparity-check codes that approach the Shannon limit as the theoreticallimit of maximum information transfer rate over a noisy channel. Thus,some available bandwidth can be increased without increasing the powerof the transmission. Instead of producing binary digits from the signal,the front-end of the decoder could be designed to produce a likelihoodmeasure for each bit.

The system, in accordance with non-limiting examples of the presentinvention, can be used in multiprocessor embedded systems and relatedmethods and also used for any type of radio software communicationsarchitecture as used on mainframe computers or small computers,including laptops with an added transceiver, such as used by militaryand civilian applications, or in a portable wireless communicationsdevice 420 as illustrated in FIG. 6. The portable wirelesscommunications device is illustrated as a radio that can include atransceiver as an internal component and handheld housing 422 with anantenna 424 and control knobs 426. A Liquid Crystal Display (LCD) orsimilar display can be positioned on the housing in an appropriatelocation for display. The various internal components, including dualprocessor systems for red and black subsystems and software that isconforming with SCA, is operative with the illustrated radio. Although aportable or handheld radio is disclosed, the architecture as describedcan be used with any processor system operative with the system inaccordance with non-limiting examples of the present invention. Anexample of a communications device that could incorporate thesimultaneous wideband and narrowband communications in accordance withnon-limiting examples of the present invention, is the Falcon® IIImanpack or tactical radio platform manufactured by Harris Corporation ofMelbourne, Fla.

The transmit and receive systems as described relative to FIGS. 1 and 2can use modified OFDM. There now follows a description of OFDM as couldbe applied to these systems as described above. In OFDM communicationssystems the frequencies and modulation of a frequency-divisionmultiplexing (FDM) communications signal are arranged orthogonal witheach other to eliminate interference between signals on each frequency.In this system, low-rate modulations with relatively long symbolscompared to the channel time characteristics are less sensitive tomultipath propagation issues. OFDM thus transmits a number of lowsymbol-rate data streams on separate narrow frequency subbands usingmultiple frequencies simultaneously instead of transmitting a single,high symbol-rate stream on one wide frequency band on a singlefrequency. These multiple subbands have the advantage that the channelpropagation effects are generally more constant over a given subbandthan over the entire channel as a whole. A classical In-phase/Quadrature(I/Q) modulation can be transmitted over individual subbands. Also, OFDMis typically used in conjunction with a Forward Error Correction scheme,which in this instance, is sometimes termed Coded Orthogonal FDM orCOFDM.

An OFDM signal can be considered the sum of a number of orthogonalsubcarrier signals, with baseband data on each individual subcarrierindependently modulated, for example, by Quadrature Amplitude Modulation(QAM) or Phase-Shift Keying (PSK). This baseband signal can alsomodulate a main RF carrier.

OFDM communications systems have high spectrum efficiency (a high numberof bits per second per Hz of bandwidth), simple mitigation of multi-pathinterference, and an ease in filtering noise. OFDM communicationssystems suffer, however, from time-variations in the channel, especiallythose which cause carrier frequency offsets. Because the OFDM signal isthe sum of a large number of subcarrier signals, it can have a highpeak-to-average amplitude or power ratio. It is also necessary tominimize intermodulation between subcarrier signals, which can createself-interference in-band, and create adjacent channel interference.Carrier phase noise, Doppler frequency shifts, and clock jitter cancreate Inter-Carrier Interference (ICI) for closely frequency-spacedsubcarriers. The subcarriers are typically transmitted at assignedfrequency locations within a transmission spectrum. Over the duration ofthe transmission of an OFDM signal, the average power per subcarrier issignificant, and can be easily detected and intercepted, which isundesirable to a system requiring Low Probability of Detection (LPD) andLow Probability of Interception (LPI) characteristics. The receiver thatis to receive the OFDM signal requires a minimum signal-to-noise ratio(SNR) per subcarrier in order to demodulate and decode the signal withan acceptably low bit error rate (BER). If there is other unwantedenergy within the transmission spectrum, the SNR can decrease causing anincrease in BER. Said unwanted energy can be unintentional noise fromother sources. In this case the noise is referred to as “interference”and the sources are referred to as “interferers.” If the unwanted energycorrupting the transmission is transmitted intentionally by some thirdparty source known as a jammer, it is referred to as a jamming signal.The conventional OFDM signal is susceptible to such interferers andjammers because of the required minimum SNR per subcarrier for anacceptably low BER. Further, frequency selective fading in the channelcauses transmission nulls within the OFDM signal's transmissionspectrum, which selectively reduce the SNR on certain subcarriers withinthose nulls, depending on their frequency location, leading to anundesirable increase in BER.

Orthogonal Frequency Division Multiplexing (OFDM) is also termedMulticarrier Modulation (MCM) because the signal uses multiple carriersignals that are transmitted at different frequencies. Some of the bitsor symbols normally transmitted on one channel or carrier are nowtransmitted by this system on multiple carriers in the channel. AdvancedDigital Signal Processing (DSP) techniques distribute the data overmultiple carriers (subcarriers) at predetermined frequencies. Forexample, if the lowest-frequency subcarrier uses a base frequency, theother subcarriers could be integer multiples of that base frequency. Theparticular relationship among the subcarriers is considered theorthogonality such that the energy from one subcarrier can appear at afrequency where all other subcarrier's energy equal zero There can be asuperposition of frequencies in the same frequency range. This resultsin a lower symbol rate on each subcarrier with less Inter-SymbolInterference (SI) due to adverse effects of multipath. In many OFDMcommunications systems, a Guard Interval (GI) or Cyclic Prefix (CP) isprefixed or appended to the OFDM symbol to mitigate the effects of ISI.

Classic OFDM is based on a frequency-division multiplexing (FDM) systemwhere each frequency channel is modulated, The frequencies andmodulation of an FDM system are now orthogonal to each other toeliminate interference between channels. Because low-rate modulationswith relatively long symbols compared to the channel timecharacteristics are less sensitive to multipath, an OFDM communicationssystem allows a number of low-rate symbol streams to be transmittedsimultaneously on multiple carriers rather than having one high-ratesymbol stream transmitted on a single carrier. Thus, the frequencyspectrum in an OFDM communications system is divided into multiplelow-bandwidth subbands. Since each subband covers a relatively narrowsection of the frequency spectrum, channel propagation effects are moreconstant or “flat” over a given subband compared to channel variationsover the entire occupied spectrum. Any type of in-phase and quadrature(I/Q) modulation can be used to modulate any subcarrier, for example,Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK)or Quadrature Amplitude Modulation (QAM), or any of the numerous anddifferent derivations of these modulation schemes. Different signalprocessing techniques, for example, channel coding, power allocation,adaptive modulation encoding, and similar schemes can be applied to oneor more subbands. Multi-user allocation is also possible for exampleusing time, coding, or frequency separation.

In a classic OFDM communications system, one transmitter will transmit asignal on dozens or thousands of different orthogonal frequencies thatare independent with respect to the relative amplitude and phaserelationship between the frequencies. Each subcarrier signal typicallywill have space for only a single narrowband signal because the signalsare closely spaced and it is important to prevent signals on adjacentsubcarriers from interfering with each other. In an OFDM system, thesymbols on each subcarrier are constructed such that energy from theirfrequency components are zero at the center of every other subcarrier,enabling a higher spectral efficiency for OFDM symbols than is possiblein classic FDM.

An OFDM system could include channel coding as a Forward ErrorCorrection (FEC) technique, using a Forward Error Correction encoder tocreate a coded orthogonal FDM (COFDM) signal. Channel-State Information(CSI) techniques can also be employed, including continuous wave (CW)interferer and/or selective channel systems.

An OFDM signal is typically the sum of each of the orthogonalsubcarriers. Baseband data is independently modulated onto each of theorthogonal subcarriers using some type of modulation, such as QuadratureAmplitude Modulation (QAM) or Phase Shift Keying (PSK) schemes asdiscussed before. Because the spectrum of each subcarrier overlaps, itcan be considerably wider than if no overlap were allowed. Thus, OFDMprovides high spectrum efficiency. Because each subcarrier operates at alow symbol rate, the duration of each symbol in the subcarrier is long.(For clarity, “symbol rate” is equal to the inverse of “symbolduration”). By using Forward Error Correction (FEC) equalization andmodulation, there can be an enhanced resistance against a) linkdispersion, b) slowly changing phase distortion and fading, c) frequencyresponse nulls, d) constant interference, and e) burst noise. Further,the use of a Guard Interval (GI) or cyclic prefix provides enhancedresistance against multipath in the transmission channel and for thecase of CROW and CE-OFDM, the guard interval allows the use of afrequency domain equalizer as shown in FIG. 2.

Typically, in OFDM communications system, a subcarrier and somewhatrectangular pulse can be employed and operative by an Inverse DiscreteFourier Transform (IDFT) using an Inverse Fast Fourier Transform (IFFT)circuit within the transmitter. At a receiver, a Fast Fourier Transform(FFT) circuit reverses this operation. The rectangular pulse shaperesults in a Sin(x)/x spectrum in the subcarriers.

The spacing of subcarriers can be chosen such that the receivedsubcarriers can cause zero or acceptably low Inter-Carrier Interference(ICI) when the receiver and transmitter are synchronized. Typically,OFDM communications systems split the available bandwidth into manynarrow-band subbands from as little as a few dozen to as many as eightthousand to ten thousand. Unlike the communications system providingmultiple channels using classical FDM, the subcarriers for each subbandin OFDM are orthogonal to each other and have close spacing and littleoverhead. In an OFDM communications system, there is also littleoverhead associated with any switching that may occur between users asin a Time Division Multiplexing Access (TDMA) communications system.Usually, the orthogonality of subcarriers in an OFDM communicationssystem allows each carrier to have an integer number of cycles over asymbol period. As a result, the spectrum of a subcarrier has a null atthe center frequency of its adjacent subcarriers.

Usually, in an OFDM communications system, the spectrum required fortransmitting data is chosen based on the input data and a desiredmodulation scheme to be used with each carrier that is assigned the datato transmit. Any amplitude and phase of the carrier is calculated basedon the modulation, for example, BPSK, QPSK or QAM as noted before. Anyrequired spectrum is converted using the IFFT circuit to ensure carriersignals are orthogonal.

It should be understood that a FFT circuit transforms a cyclic timedomain signal to an equivalent frequency spectrum by finding anequivalent waveform that is generated as a sum of orthogonal sinusoidalcomponents. The frequency spectrum of the time domain signal is usuallyrepresented by the amplitude and phase sinusoidal components. The IFFTcircuit performs the reverse process and transforms the spectrum of theamplitude and phase into a time domain signal. For example, an IFFTcircuit can convert a set of complex data points into a time domainsignal of the same number of points. Each complex input point willresult in an integral number of sinusoid and cosinusoid cyclesrepresented by the same number of points as were input to the IFFT. Eachsinusoid known as the in-phase component, and cosinusoid known as thequadrature component, will be orthogonal to all other componentsgenerated by the IFFT. Thus, orthogonal carriers can be generated bysetting an amplitude and phase for each frequency point representing adesired subcarrier frequency and performing the IFFT.

It should be understood that a Guard Interval (GI) or Guard Time, alsotermed a cyclic prefix, often is added to an OFDM symbol. The guardinterval reduces the effects of the wireless channel on Inter-SymbolInterference (ISI) and contains redundant transmission information.Referring to the IEEE 802.11a standard as a non-limiting example, if acarrier spacing is 312.5 KHz, and the Fourier Transforms are performedover 3.2 microseconds, then a 0.8 microsecond guard interval can beapplied for ISI rejection. The guard “interval” could be the last T_(g)seconds of an active symbol period that is prefixed to an OFDM symbol,making it a cyclic prefix. It is kept short for a fraction of “T,”corresponding to the total length of the active symbol, yet longer thanthe channel impulse response. This helps reduce the ISI andInter-Carrier Interference (ICI) and maintains subcarrier orthogonality.A time waveform appears periodic to the receiver over the duration ofthe FFT.

To reduce ICI, the OFDM symbol can be cyclically extended in the guardtime to ensure that delayed replicas of the OFDM symbol can have aninteger number of cycles within the FFT interval, as long as the delayis smaller than the guard time. As a result, multipath signals withdelays smaller than the guard time would not produce ICI.

Multipath interference is caused when multiple copies of the transmittedsignal arrive at the receiver at different times. It should beunderstood that an OFDM communications system reduces the effect ofmultipath interference by providing the ability to add signal redundancyin both frequency and time by the use of various coding algorithms. Forexample, with the IEEE 802.11a standard using OFDM, 48 carriers can betransmitted simultaneously. The coding gain can be provided using aone-half (½) convolutional encoder at the transmitter and later aViterbi decoder. Data bits can be interleaved across multiple symbolsand carriers. Lost data often is recoverable because of interleavingacross the frequency and time space.

Increasing the data rate requires an increase in the symbol rate for afixed number of carriers, fixed modulation scheme and fixed sample rate.For a single carrier system, complex equalizers and adaptive filters arerequired at the receiver to compensate for the magnitude and timedistortions caused by the channel. The accuracy and dynamic rangerequired of such equalizers and filters increases markedly as symboltimes are decreased. However, in an OFDM system, for example, when 48subcarriers are transmitted simultaneously, the symbol rate iseffectively reduced by 48 times, significantly reducing the requirementsof channel equalizers and filters. The reduced symbol rate of an OFDMsystem enables a robust communication link, resistant to IST.

It should be understood that an OFDM receiver receives a sum of thedifferent signals as subcarriers. The addition of a guard interval canfurther enhance performance in an OFDM system by ensuring that no symboltransitions occur during each received symbol time. For example, if anOFDM subcarrier is BPSK modulated, there would be a 180 degree phasejump at symbol boundaries. By choosing a guard interval that is longerthan the largest expected time difference between the first and lastmultipath signals, such phase transitions can occur only during theguard time, meaning there are no phase transitions during the FFTinterval. If the phase transitions of a delayed path occur within theFFT interval of the receiver, then the summation of the subcarriers ofthe first path with the phase modulated waves of the delayed path wouldno longer produce a set of orthogonal subcarriers, resulting in acertain level of interference.

A Walsh transform can be applied to spread subcarriers over thefrequency domain, in contrast with spreading over the time domain aswith conventional CDMA systems. Applying a Walsh transform before anyIFFT circuit can reduce average power for enhanced LPI/LPD. Variousaspects of the communications system can be readily varied for improvedperformance. With fewer subcarriers as compared to the IFFT size and thespreading sequence length, more processing gain may be realized fromfrequency domain spreading. Furthermore, LPI/LPD and Anti-Jamming (AJ)performance can be enhanced, and there can be higher SNR per subcarrier.Increasing the sample rate also increases the bandwidth and data rate,and improves the LPI/LPD/AJ performance.

A Frequency Domain Spreader circuit typically operates as a matrixoperation. For example, if a 64 IFFT circuit is employed, then a 64×64Walsh Matrix (as a non-limiting example) can be used to frequency-spreadthe subcarriers and provide processing gain. An input vector would bemultiplied by the Walsh matrix. It should be understood that a Walshmatrix is a square matrix with dimensions that can be a power of “two.”The entries are positive or negative one (+1, −1). The Walsh matrix canbe obtained from a Hadamard Matrix that is defined by a recursiveformula of the same dimension by arranging rows such that the number ofsign changes is in increasing order, i.e., sequential ordering. Each rowof a Walsh matrix corresponds to a Walsh function. The ordering of rowsin a Walsh matrix can be derived from ordering a Hadamard matrix byapplying a bit-reversal permutation and a Gray code permutation. TheWalsh functions form an orthogonal basis of a square that isintegratable on a unit interval. Thus, it can generate statisticallyunique sets of numbers suitable for use in encryption, also known as“pseudo-random and noise codes.” The multiplication may be implementedefficiently as a series of additions and subtractions.

This application is related to copending patent application entitled,“SYSTEM AND METHOD FOR COMMUNICATING DATA USING CONSTANT RADIUSORTHOGONAL WALSH MODULATION,” which is filed on the same date and by thesame assignee and inventors, the disclosure which is hereby incorporatedby reference.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

1. A radio device, comprising: a transmitter unit comprising a modulatorconfigured to generate M-PAM (M-Pulse Amplitude Modulation)communications symbols containing communications data; a Fast WalshTransform (FWT) circuit connected to the modulator and configured toreceive the communications symbols from the modulator and orthogonallyencode and band-spread the communications symbols, or channels, usingthe Fast Walsh Transform; a guard time circuit connected to the FastWalsh Transform circuit and configured to cyclically extend the outputof Fast Walsh Transform circuit by a guard time; and a frequencymodulation circuit connected to the guard time circuit and configured tofrequency modulate the communications symbols wherein a constantenvelope orthogonal Walsh modulated communications signal is generatedhaving a plurality of orthogonal waveforms each forming a separate Walshcommunications channel.
 2. The radio device according to claim 1,wherein at least one of said separate Walsh communications channelscomprises a broadcast channel for transmitting a communications signalcontaining communications data to a plurality of receivers.
 3. The radiodevice according to claim 1, wherein at least one of said separate Walshcommunications channels comprises at least one of a voice and datachannel.
 4. The radio device according to claim 1, wherein at least onesaid separate Walsh communications channels comprises a control channel.5. The radio device according to claim 1, wherein a substantial portionof transmit power from the transmitter unit can be allocated to at leastone of the Walsh communications channels.
 6. The radio device accordingto claim 1, and further comprising at least one of a square root raisedcosine filter operative before the frequency modulation circuit forband-limiting a signal containing the communications symbols and aclipping device operative before the frequency modulation circuit forreducing the peak to average power ratio of signal.
 7. The radio deviceaccording to claim 1, wherein said modulator comprises a circuit forgenerating binary or multilevel M-PAM communications symbols.
 8. Theradio device according to claim 1, and further comprising an up-samplingcircuit that receives the signal from the FWT circuit for increasing thesampling rate on the signal containing the communications symbols.
 9. Asystem for communicating data, comprising: a transmitter comprising amodulator configured to generate M-PAM (M-Pulse Amplitude Modulation)communications symbols, or channels, containing communications data; aFast Walsh Transform (FWT) circuit connected to the modulator andconfigured to receive the communications symbols from the modulator andorthogonally encode and band-spread the communications symbols using theFast Walsh Transform; a guard time circuit connected to the Fast WalshTransform circuit and configured to cyclically extend the output of FastWalsh Transform circuit by a guard time; a frequency modulation circuitconnected to the guard time circuit and configured to frequency modulatethe communications symbols, or channels, wherein a constant envelopeorthogonal Walsh modulated communications signal is generated having aplurality of orthogonal waveforms each forming a separate Walshcommunications channel; an antenna through which the constant envelopeorthogonal Walsh modulated communications signal is transmitted; and atleast one receiver remote from the transmitter that receives theconstant envelope Walsh modulated communications signal and comprisingat least one of a phase demodulator circuit configured to phasedemodulate the received signal, a phase unwrap circuit configured tophase unwrap the received signal, a guard time removal circuit and a FWTcircuit configured to perform a Fast Walsh Transform as a demodulatormatched filter to obtain any communications data that had beentransmitted from the transmitter and contained within the at least onereceived Walsh communications channel.
 10. The system according to claim9, and further comprising a plurality of receivers remote from thetransmitter and wherein at least one of said separate Walshcommunications channels comprises a broadcast channel for transmitting acommunications signal containing communications data to the plurality ofreceivers.
 11. The system according to claim 9, wherein at least one ofsaid separate Walsh communications channels comprises at least one of avoice and data channel.
 12. The system according to claim 9, wherein atleast one said separate Walsh communications channels comprises acontrol channel.
 13. The system according to claim 9, wherein asubstantial portion of transmit power from the transmitter is allocatedto at least one Walsh communications channel.
 14. The system accordingto claim 9, wherein said transmitter further comprises at least one of asquare root raised cosine filter operative before the frequencymodulation circuit for band-limiting a signal containing thecommunications symbols and a clipping device operative before thefrequency modulation circuit for reducing the peak to average powerratio of signal.
 15. The system according to claim 9, wherein saidmodulator of said transmitter further comprises a circuit for generatingbinary or multilevel M-PAM communications symbols.
 16. The systemaccording to claim 9, wherein said receiver further comprises amulti-user detection circuit for applying standard or iterativemulti-user detection algorithms to the received signal.
 17. The systemaccording to claim 16, wherein said receiver further comprises afeedback loop from the multi-user detection circuit for iterativeprocessing.
 18. A method of communicating data, comprising: generatingM-PAM (M-Pulse Amplitude Modulation) communications symbols containingcommunications data; receiving the modulated communications symbols andorthogonally encoding and band-spreading the communications symbolsusing a Fast Walsh Transform; cyclically extending the Fast WalshTransform output by a guard time; frequency modulating thecommunications symbols to generate a constant envelope orthogonal Walshmodulated communications signal having a plurality of orthogonalwaveforms each forming a separate Walsh communications channel; andtransmitting the constant envelope orthogonal Walsh modulatedcommunications signal to at least one receiver.
 19. The method accordingto claim 18, wherein at least ohe of the separate Walsh communicationschannels comprises a broadcast channel that is transmitted to aplurality of receivers.
 20. The method according to claim 18, whichfurther comprises transmitting at least one of voice and data on aseparate Walsh communications channel.
 21. The method according to claim18, which further comprises using one of the separate Walshcommunications channels as a control channel.
 22. The method accordingto claim 18, which further comprises allocating a substantial portion oftransmit power from the transmitter to at least one Walsh communicationschannel.
 23. The method according to claim 18, which further comprisesband-limiting the signal by filtering the signal within a square rootraised cosine filter before frequency modulating the communicationssignal.
 24. The method according to claim 18, which further comprisesreceiving the constant envelope orthogonal Walsh modulatedcommunications signal within a receiver and phase demodulating thesignal, removing the guard time and performing a Fast Walsh Transform asa demodulator matched filter to obtain the communications data.
 25. Themethod according to claim 24, which further comprises processing thereceived communications signal at the receiver using standard oriterative multi-user detection (MUD) algorithms.